1. Field
The present invention relates to a WDM signal monitoring system that monitors a power level and a wavelength of each channel of a wavelength division multiplexed (WDM) signal, that is, a so-called optical channel monitor (OCM), and a WDM transmission device equipped with the WDM signal monitoring system.
2. Description of the Related Art
The WDM transmission device measures a power level and a wavelength of a Wavelength Division Multiplex (WDM) signal by Optical Channel Monitor (OCM). Examples of construction of the OCM are shown in FIGS. 1 and 2. An example in FIG. 1 demultiplexes a WDM signal by an Arrayed Waveguide Grating (AWG) divider 10, and receives it with a photodiode (PD) array 12. In addition, an example in FIG. 2 spatially performs dispersion using a diffraction grating 14, and receives it with a PD array 16.
Regarding an OCM, in order to reduce cost, it is desirable to reduce the number of PDs. In the example of FIG. 1, four PDs per each channel are used as shown in an enlarged view on a right-hand side in FIG. 1. Then, the example finds a spectrum waveform by performing approximate calculation, such as Gaussian approximation, on the basis of a received light level of each PD, and determines the power level and wavelength of the signal. This approximation procedure is shown in FIG. 3. In FIG. 3, black dots 18 denote measured values from four PDs, and a curve 20 shows an approximated curve. By performing approximation as shown in the diagram, the power level P and wavelength λ of the channel are determined.
As an example, waveforms of 10-Gbps signal and 40-Gbps signal at the time of adopting a Non Return to Zero (NRZ) modulation system are shown in FIGS. 4 and 5, respectively. In this way, waveforms of a signal are different when bit rates of the signal are different.
Therefore, there is a problem that accuracy of an approximate calculation of OCM deteriorates and the power level and wavelength that are measured shift from actual values. In addition, as exemplified as a schematic diagram in FIG. 6, since spectral shapes change also by modulation systems, there is the same problem. Therefore, since the power level of the WDM signal is controlled on the basis of the wrong power level information, tilt compensation is not fully performed.
In addition to the above-mentioned problems, since the number of filters and nonlinear effects that are passed is different also with a route along which a path of an optical signal received has already passed, there is a problem that a spectrum waveform transforms.